Addressing techniques or multiplexing schemes for liquid crystal array devices are known. Typically, the array will comprise a first plurality of electrodes arranged parallel to each other on a first substrate of the device and the second plurality of electrodes arranged parallel to each other, but perpendicular to the first plurality of electrodes, on the second substrate of the device. A plurality of liquid crystal pixels are thus defined at the point where these perpendicular electrode structures intersect. Because each liquid crystal pixel does not have its own unique electrode connections some form of multiplexing is required to address the pixels of the device. Usually a first signal, known as a strobe signal, is applied in succession to each of the first plurality of electrodes while a second signal is applied to each of the second plurality of electrodes. Thus, when the strobe signal is applied to a given electrode (hereafter referred to a row electrode) data signals may be applied to the second plurality of electrodes (hereafter referred to as column electrodes) to control the state of the pixels in that row.
One such multiplexing scheme, applied to ferroelectric liquid crystal displays, is described in the "JOERS/ALVEY Ferroelectric Multiplexing Scheme published in Ferroelectrics 1991, Volume 122, pages 63 to 79. In the scheme described in this prior art reference the plurality of second signals comprise either a first or second data waveform. The first data waveform comprises a positive-going rectangular wave immediately followed by a negative-going rectangular wave of the same amplitude and duration. The second data waveform is the inverse of the first.
In a liquid crystal device array which is addressed using such a multiplexing scheme the column (data) waveforms are applied to all of the pixels in their respective columns regardless of whether those pixels are actually being addressed. In other words the column waveforms are applied to the pixels of the device which are not receiving a strobe signal at that moment. When the array device is a ferroelectric liquid crystal (FLC) array the application of these waveforms is required to provide AC stabilization of the liquid crystal material in the device. As its name suggests, AC stabilization comprises an alternating signal applied to pixels which do not currently have a strobe signal applied to them. The stabilization is applied to provide improved brightness and contrast in a display device as is well known in the art.
These waveforms cannot be removed by, for example, arranging for the row driving circuitry to be open-circuit when a strobe signal is not applied to a particular row. The voltage of the floating row electrode would effectively be at a level specified by an average of the voltage applied to the columns. For example, if all of the column electrodes have a voltage V applied then the row electrode will also be at a voltage V resulting in zero potential across the liquid crystal in that row and no AC stabilization. However, if some of the column electrodes have a voltage V applied and some have a voltage -V applied then the row voltage would be at an intermediate level and some AC stabilization would be effected. As the contrast ratio and brightness are a function of the AC stabilization voltage this technique could reduce the total power consumed by the panel but would generally lead to a spatial and temporal variation in image quality.
Such liquid crystal device arrays, particularly large area liquid crystal arrays, provide not inconsiderable driving problems because they comprise a large number of capacitors (the pixels) connected by a series string of resistors (the electrodes). The AC waveforms applied to the column electrodes thus have to drive a distributed RC ladder at high frequency. This causes power dissipation in the resistances and the liquid crystal array device warms up. This causes a particular problem in ferroelectric liquid crystal array devices which are much more sensitive to temperature than, say, an equivalent nematic liquid crystal device.